EXCEEDTHE SPACE PROVIDED. The aims of this project are to analyze the development and maturation of neural substrates for auditory information processing in the brainstem of the bullfrog, Ratio catesbeiana, across metamorphic development. Metamorphosis involves the reorganization of the auditory system from one optimized for detecting underwater sound to one more sensitive to airborne sound; anatomical, molecular, and functional changes are involved in this reorganization. This proposal outlines a series of experiments usingphysiological, anatomical and immunohistochemical techniques designed to test specific hypotheses about the emergence of auditory function across metamorphosis. Physiological experiments will focus on acoustic information processing in the dorsal medulla, focusing on the dorsolateral (acoustic) nucleus (DLN). We expect to find significant changes in both spectral and temporal coding of sounds in this nucleusover metamorphic development. We hypothesize that changes in neural response properties in the dorsal medulla over development reflect the progressive and sequential maturation of the acoustic receptor organs (saccule, amphibian papilla, basilar papilla) in the inner ear. We will use anatomical techniques to characterize changes in cell morphology and neural connectivity of the dorsal medulla across metamorphic development. Tract-tracing experiments will detail the projections of the frog's 3 auditory receptor organs to the dorsal medulla.,and how these projections mightreorganize overlarval development. Immunohistochemical procedures will detail the expression of the putative inhibitory transmitters GABA and glycine in the brainstem over development. Expression of these two molecules will be correlated with changes in excitatory and inhibitory neural response properties of DLN neurons, and the appearance of adult-like connectivity. In addition, molecular markers will be used to analyze patterns of cell birth, cell death, neurite outgrowth, and synaptogenesis as the dorsal medulla reorganizes in preparation for adult life. As there are environmental, physiological and anatomical parallels between metamorphosis and mammalianintrauterine sensory development, our findings from the frog will be useful for understanding early auditory development in humans.